Electrically conductive coatings are useful in a variety of applications, such as in EMI/RFI shielding for electronic devices, conductive coatings for electrodes in medical devices such as electrocardiograph (EKG) electrodes and conductive coatings for corrosion prevention. In the industry today, the most widely used conductive coating compositions are solvent based which have a potential of polluting the environment. The need to reduce pollution from volatile organic chemicals (VOC) have led the coating industry and government agencies to push for low VOC thick film coating compositions such as water based thick film coating compositions.
In the prior art of water based electrically conductive coating compositions, polymer latex and aqueous solutions of water soluble polymers are commonly used. U.S. Pat. Nos. 4,715,989 and 4,950,423 describe water based conductive compositions containing metal powders, latex of acrylic copolymer and aqueous dispersion of polyurethane. U.S. Pat. No. 5,286,415 describes an aqueous polymer thick film composition comprising metal particles, a water-soluble thermoplastic polymer and a polymer dispersion. U.S. Pat. No. 5,492,653 describes an aqueous silver composition having a water-soluble polymer as the polymeric binder. Dried coatings of the above-described electrically conductive compositions suffer from sensitivity to water and alcohol due to the presence of water-soluble polymers or hydrophilic components in the polymeric binders. These conductive compositions generally also have low coating hardness due to the use of thermoplastic polymers of relatively low glass transition temperature Tg. Therefore, there remains a need for a water based coating composition that can provide an electrically conductive coating with good solvent resistance and good scratch resistance. The present invention fills that need.